Semiconductor lasers have gained influence in high power laser applications because of their higher efficiency, advantages in SWAP (size, weight and power) and their lower cost over other forms of high power lasers. Many imaging applications require high power illumination such as structured light sources for 3D imaging, LADAR, Time of Flight (TOF) 3D imaging, aviation defense, and fusion research, among others. At present, edge emitting semiconductor lasers or their arrays virtually dominate sales in these fields due to their high power outputs. In the communications field, however, Vertical Cavity Surface Emitting Array (VCSEL) devices have largely replaced edge emitting devices due to the low power applications and high frequency superiority and manufacturing advantages of VCSEL devices (VCSELs) over edge emitter (EE) devices.
VCSELs can be manufactured in arrays much more cost efficiently than EE arrays, but as the area of the VCSEL array grows, heat dissipation from all of the elements can severely limit operational characteristics. Many VCSELs currently use back emitting configurations in order to distribute heat quickly to a heat sinking substrate for improved operation. If arrayed elements are not heat sunk properly, the combined heating effects will have extreme detrimental effects on the properties and lifetime of the photonic devices.
Existing top emitting VCSEL arrays have heat sinking around the VCSEL or covering the VCSEL. This heat sinking can remove heat from the immediate area, but this configuration requires heat to be transported back through the VCSEL substrate, which is formed of Gallium Arsenide (GaAs), which is a poor thermal conductor (0.55 W cm-1° C.-1). This in effect traps the heat being generated on or near the surface, thereby quickly “heating” such devices up to elevated temperatures, which begins to severely impair operational characteristics. VCSEL devices can be fabricated with very high density which exasperates the problem. Top emitting VCSEL designs are especially vulnerable to this problem. VCSEL array devices with wavelengths below ˜900 nm most likely are designed to be top emitters as the wavelength will not travel through the substrate due to the GaAs transmission curve.
Many applications require wavelengths lower than 900 nm to illuminate in range sensitive to inexpensive CMOS camera or sensor technology. Illumination from an infrared (IR) or Near infrared (NIR) source is important as the camera can be used in conjunction with narrow bandpass filters or similar pass filters to reduce and other stray or ambient light. This allows only the specific laser wavelength to be detected. Illumination of the field of view can require high powers which in turn require high density arrays of top emitters. As explained earlier, the thermal constraints of such arrays limit their output so the current technology is insufficient for many of the emerging applications.
Fabrication methodologies for VCSEL device arrays as compared to EE array fabrication offer multiple benefits including a much higher level of manufacturability using photolithographic techniques rather than mechanical means to produce multiple emitters in a 2D formation. This makes the VCSEL array emitters very useful for larger arrays, but the heating constraints of the top emitting VCSEL arrays limits operational characteristics so the top emitting VCSEL array designs have not been utilized for such applications due to this problem. It is therefore desirable to develop embodiments of top emitting VCSEL arrays that can operate at much higher outputs and performance because this would enable many new and more powerful illumination, scanning or data transmission devices. VCSEL arrays, if thermally managed properly, could also have benefits above other multibeam semiconductor arrays, including the benefits of VCSEL beam quality, reliability, modulation flexibility, and cost efficiency that would enable VCSEL devices to compete with edge emitting (EE) semiconductor arrays for many applications.
A variety of VCSEL devices and methods for manufacturing such devices are known. See, for example, U.S. Pat. Nos. 5,359,618 and 5,164,949. Forming VCSELs into two-dimensional arrays for data displays is also known. See, U.S. Pat. No. 7,957,497 B2 and U.S. Patent Publication No. 2006/0109883 A1, although the majority of these arrays are for display applications, wavelength division multiplexing, or 2D arrays for individually addressable parallel communications. Flip Chip Multibeam VCSEL arrays for higher output power have been mentioned, such as in U.S. Patent Publication No. 2006/0109883 A1, where the heat can be transferred to a heat sink substrate when using a back emitting design. However, as shown in FIG. 8B of the same document, when using a top emitting design, there is no transfer to a separate heat sink substrate except using the path through the poor thermally conductive device substrate.
The above discussion illustrates the overwhelming problem with top emitters, as the laser emission would be blocked if a substrate were to be placed over the top of the mesas. In particular, top emitting arrays with heat sinking is mentioned in U.S. Pat. No. 7,680,168. However, as noted therein, the heat sinking occurs locally and is not removed from the top of the mesas. As a result, the device distributes the heat to the VCSEL substrate which in turn must transfer the heat through a poor thermal conductive GaAs substrate. This design allows heat build-up at the surface of the higher conductive materials around the actual devices. This heating of the devices contributes to a lower, output power, lower reliability, and a lower frequency response as the junction temperature rises. Also, in current designs, there is a high capacitance problem due to the thin dielectric separation of the P type contact and the N substrate. This high capacitance device dramatically lowers the frequency response. These problems have not been adequately addressed in prior designs.
Semiconductor devices such as lasers, photodetectors, modulators, high electron mobility transistors, resonant tunneling diodes, heterojunction bipolar transistors, and the like, are often used in high-frequency applications. In many cases it is desirable to operate these optoelectronic and electronic devices at higher powers, and higher frequencies with minimal eat generation. It is well known that heat generation negatively affects device lifetime, performance, and the rate of data transmission. Arraying devices together increases heat generation adding to the negative effects on power, lifetimes and high frequency operation. It would therefore be desirable to dissipate heat and reduce parasitic capacitance from a photonic semiconductor device.